The Non-Dispersive Infrared (“NDIR”) technique has long been considered as one of the best methods for gas measurement. In addition to being highly specific, NDIR gas analyzers are also very sensitive, stable, reliable and easy to maintain and service. Ever since the NDIR technique of gas measurement was first introduced and practiced in the mid 1950's, a large number of improved measurement techniques based upon the NDIR principle for gas detection have been proposed and successfully demonstrated. The most notable advances over the years in this field are summarized as follows.
Burch et al. (U.S. Pat. No. 3,793,525) and Blau et al. (U.S. Pat. No. 3,811,776) in 1974 were the first to advance a so-called “Double Beam” technique for NDIR gas measurement by taking advantage of the principle of nonlinear absorption for some strongly absorbing gases such as CO2 to create a reference channel. Shortly thereafter, this “Double Beam” NDIR gas sensor technique was greatly simplified with the use of two interposed spectral filters (one absorbing and one neutral) to create a sample and a reference detector channel. Subsequent NDIR gas sensors, designed using this technique, have enjoyed good performance alluded to briefly above.
In U.S. Pat. No. 4,578,762 (1986) Wong advanced the first self-calibrating NDIR CO2 analyzer using a novel two-wheel chopper and mirror arrangement. Another improved type of such gas analyzer is shown and described in U.S. Pat. No. 4,694,173 (1987) by Wong. This gas sensor has no moving parts for effecting the interposition of spectral filters to create both a sample and reference detector channel as in the NDIR gas analyzers described earlier.
In U.S. Pat. No. 5,163,332 (1992), Wong advanced the so-called “wave-guide” sample chamber concept for simplifying NDIR gas sensors into ones that are compact, rugged and low-cost while still maintaining their superior performance characteristics. This concept has subsequently been widely adopted in the design of today's NDIR gas sensors, particularly in low-cost and high volume versions.
All of the NDIR gas analyzers described above for the measurement of the concentrations of one or more gases in a mixture perform well functionally and have contributed successfully to the overall technical advancement in the field of gas analysis during the past two decades. They have been widely accepted in both the medical and industrial communities. Despite their undisputed success over the years, there still remain a number of important sensor performance characteristics that need to be greatly improved in order to further extend the useful applications of these devices in a number of areas.
By far the most deficient performance characteristic of gas sensors of today, inclusive of NDIR gas sensors, is the sensor output stability over time. Unlike the temperature controller or thermostat device which just about everybody is familiar with at home or in their workplaces for sensing temperature that never requires output adjustment or recalibration over time, such is not the case for gas sensors irrespective of their operational principle, functional design, material construct or even costs. Dependent upon the type of gas sensors, just about every one of them requires recalibration once every six months to a year without exception in order that they remain accurate over time. While this performance deficiency has been well tolerated over the years, it remains as a significant drawback for gas sensors and even precludes their use in a number of vital applications and therefore there has been a long-felt need for elimination of this problem.
The second most prominent performance deficiency for gas sensors of today irrespective of their operational principle is their output dependence as a function of the temperature of the environment wherein the sensors are located. This performance deficiency for just about all gas sensors is universally, albeit reluctantly, dealt with by specifying the output correction per degree of temperature change with respect to the output stipulated at a standard temperature. In some gas sensors these output temperature corrections are quite large and in many cases severely limit the use of these sensors outdoors. It would be a significant step forward in the development of future gas sensors, particularly for the NDIR type, because of its prevalent use in most industries, that this performance deficiency be also overcome and, again, there has been a long-felt need for overcoming this problem.
The afore-mentioned serious NDIR gas sensor performance deficiencies, namely sensor output drift over time and output dependency as a function of exposed sensor temperature, have earlier been addressed by the present inventor in a provisional patent application 61/274,874 to the US Patent Office filed on Aug. 21, 2009 and entitled “Absorption Biased NDIR Gas Sensing Methodology.” In this recent patent disclosure, the present inventor takes advantage of the fact that if the spectral content of radiation from the source and/or convoluted with those from the surroundings be always kept the same for both the reference and the signal channels of an NDIR gas sensor, assuming that this sensor uses the most widely deployed dual-channel methodology, the output of the sensor taken as the ratio of the signal output over the reference output can always be kept constant or unchanged over time except when the gas of interest is present in the sample chamber.
In order that this recently disclosed Absorption Biased methodology be implemented, both the signal and the reference channel must be provided with exactly the same spectral narrow band pass filter designed for detecting the gas of interest in front of the respective infrared detectors. In order to differentiate between the signal and the reference channel outputs from the respective detectors in the presence of the gas of interest, an absorption bias is designed between the two channels via the use of different sample chamber path lengths for the two channels. Thus, if the sample chamber path length for the signal channel is longer than that for the reference channel, the signal channel detector output will change greater (or be reduced more) than that for the reference channel when the same concentration level of the gas of interest is present in the sample chamber. In other words, the sensor output will change as the concentration level of the gas of interest changes in the sample chamber as reflected by the calibration curve which can be prepared for the sensor.
The fact that both detection channels have the same narrow band pass spectral filter and they receive radiation from one and the same single infrared source as taught by the widely deployed dual-channel NDIR gas detection methodology, they are all affected in the same way to first order when there are spectral changes caused by temperature variations in the sample chamber and/or by the short or long-term operational changes (e.g. aging) of the infrared source. Thus the outputs of the dual-channel NDIR gas sensor for the detection of any gas of interest implemented using the inventor's recently disclosed Absorption Biased methodology will stay virtually drift-free over time without the need for any periodic re-calibration or software correction.
While the Absorption Biased NDIR gas sensor methodology as disclosed recently by the present inventor adequately addresses the serious deficiencies of output instability over time and dependence of sensor temperature for presently deployed dual-channel NDIR gas sensors, two other important performance characteristics for these sensors, namely miniaturization and lowest possible unit cost, have still not been taken into consideration. Electrochemical gas sensors have long been considered to be small and low cost, but their performances are also known to suffer from output instability over time and relatively short operating lives when compared to other non-electrochemical gas sensors, particularly the NDIR types. Over the past several years, the advent of MEMS (Micro Electro-Mechanical System) sensors of all types have steadfastly driven the available sensor sizes drastically downwards. However, such a promising sensor technology with its own limitations has yet to penetrate into the domain of NDIR gas sensors. The requirements for stable infrared sources and detectors along with physically long path length sample chambers in order to detector very low concentration of interested gases are still impediments for the MEMS technology to overcome in the future.
The realization of NDIR gas sensor miniaturization achieving concomitantly the lowest possible unit sensor cost has long been the goal of many R&D engineers working in this field. Over a decade ago, Wong disclosed in U.S. Pat. No. 5,444,249 (Aug. 22, 1995) a miniaturized NDIR gas sensor manufactured using semiconductor micro-machining techniques from a semiconductor material such as Si or GaAs. The NDIR gas sensor comprises an optical waveguide, a light source at one end of the waveguide, at least one light detector at the end of the waveguide opposite the light source. A diffusion type gas sample chamber is formed within the waveguide and interposed in the optical path between the light source and the light detector. The light source and light detector, with a separate band pass filter interposed between them, are thermally isolated from the waveguide which acts as the gas sample chamber for the sensor. Since the NDIR sensor is fabricated out of a semiconductor material, the source driver and signal processing electronics can be added directly to the sensor using integrated circuit fabrication techniques. Unfortunately, such a proposed miniaturized NDIR gas sensor is only a single-channel device whose performance is far inferior to the present-day deployed dual-channel sensors even before their deficiencies are remedied by the recently disclosed Absorption Biased technique.
From the above discussion, it is quite apparent that the recently disclosed Absorption Biased methodology for NDIR gas sensors can further be improved with an innovative approach for miniaturization and a concomitant realization of achieving a lower unit cost for such a sensor.